Dental copings or posts coated with a dental porcelain have been used in restorations of single or multiple teeth. The jacket or covering of dental porcelain on the metal coping or post provides a restoration which closely resembles a natural tooth. Dental crown and bridge restorations are often made with a metal base comprising a malleable metal alloy which is gentle on opposing dentition. Both types of restorations are well known and have been used for many years.
The general technique for the construction of a porcelain coated dental restoration involves first taking an impression of a denture area that has been prepared to receive the restoration. A die is prepared from the impression, and a metal base ("coping" or "post") is cast to fit the die. The metal base has an internal shape to match the prepared denture. A porcelain powder is then mixed with water to form a slurry which is then applied to the metal base, or a portion of a metal base, by standard procedures. The slurry is shaped in the form of the finished tooth, crown, multiple unit bridge, inlay or onlay. Alternatively, a resin-containing composite may be used to form the outer coating of the restoration. The porcelain is then dried, and fired in a furnace at a desired firing temperature. The restoration may be fired several times before the final form is obtained, and the porcelain may be applied in several layers.
To form a strong bond between a dental porcelain and a dental alloy, it has been determined that the temperature range in which the glass-forming components of a porcelain composition melt and mature should be slightly below the melting point of the dental alloy. Herein, the temperature or temperature range in which the glass-forming particles in the porcelain mixture melt to form a glass melt is referred to as the fusion temperature or the fusion temperature range. To form a strong bond, it is also important that the thermal expansion coefficient of the porcelain be close to, but slightly less than, that of the dental alloy.
There is a significant temperature change when heating a restoration from room temperature to the firing temperature, as occurs when a restoration is alternately fired and cooled. Therefore, significant stress can be induced in the restoration if the thermal expansion of the porcelain coating does not closely match that of the dental alloy base.
Crown and bridge alloys have different properties when compared to traditional ceramic alloys used with a fused porcelain veneer. Crown and bridge alloys have a lower casting temperature and a higher coefficient of thermal expansion than traditional ceramic alloys. Traditional porcelains would not bond strongly to crown and bridge alloys due to the large difference in thermal expansion coefficients of the alloys and the porcelains. It is desirable to develop a dental restoration system which provides the convenience of a low casting temperature and high expanding alloy with a dental porcelain having a correspondingly high thermal expansion coefficient.
Table I below shows the melting or fusion temperature range of traditional ceramic alloys, traditional dental porcelains and crown and bridge alloys.
TABLE I ______________________________________ MELTING/FUSION COEF. of THERM. TEMP. RANGE EXP. (in/in/.degree.C.) (.degree.C.) (RT-500.degree.C.) ______________________________________ CERAMIC 1050-1300 13.6-15 .times. 10.sup.-6 ALLOYS TRADITIONAL 925-1000 12.7-14 .times. 10.sup.-6 PORCELAIN CROWN AND 815-1000 15.5-18.3 .times. 10.sup.-6 BRIDGE ALLOYS ______________________________________
Recently, many dental porcelains have been developed which exhibit high coefficients of thermal expansion. To be successfully used in dental restorations, such porcelains must be used with correspondingly high expanding dental alloy bases or copings. It is desirable to provide a family of dental alloys having coefficients of thermal expansion which exceed 15.times.10.sup.-6 in/in/.degree. C. when heating from room temperature to 500.degree. C. and which can be used with a wide variety of high thermal expansion dental porcelains.
Dental alloy bases that are most often employed today in such restorations include gold, high and low gold alloys including gold-palladium alloys, silver-palladium alloys, high palladium alloys, nickel-chrome-molybdenum type alloys, gold-silver-palladium alloys and palladium-copper alloys. Gold and its alloys are preferred metals for a metal base due to their biocompatibility with the human body. Precious metal alloys exhibit thermal expansion coefficients of about 13.6.times.10.sup.-6 to 18.3.times.10.sup.-6 in/in/.degree. C. and thus ceramics which are used with gold metal and alloys should have similar thermal expansion coefficients. Many additives have been used to alter and improve the properties of gold and gold alloys. Among such additives .are palladium, gallium and copper.
Recently, there has become a heightened concern among dentists and patients as to whether alloys comprising palladium, gallium or copper are completely safe. It is feared that these elements may not be as biocompatible with the human body as once thought. While there is no scientific proof of any unsafe qualities of dental alloys containing these elements, it is nonetheless desired by some dentists and patients to use dental alloys which do not contain these elements.